Getting refused entry and removed at Heathrow, X in passport

Terminology-wise: You applied for 'leave to enter' under Appendix V of the Immigration Rules at a UK port and were refused. You were subsequently removed from the UK. It's an administrative procedure that takes place when the IO determines that a person is in violation of Paragraph 320 of the Immigration Rules (the fact that entry is being sought for a purpose not covered by these Rules). All removals from port take place under Paragraph 320. Note that there is an important distinction between removal and deportation; you were not deported.

What will happen now?

The UK will send your biometrics to the other members of the affluent Commonwealth along with the DHS and to the Schengen members. They do that because there is an agreement with those countries to share information.

Most often people are upset about the wasted airfares and the duress of being in detention while the paperwork is being sorted out.

It happens all the time and it's an upsetting experience. During the removal they gave you some paperwork, hang on to it. We don't need to see the paperwork for removal cases (those are very straightforward), but it's likely they got you on V 4.2 (b) "will not live in the UK for extended periods through frequent or successive visits, or make the UK their main home" You should apply for entry clearance.

Before I was seeking entry as a visitor when I got refused and I want to visit again before getting this partner visa.

You need to create an account at Visa4UK and apply for entry clearance as a 'standard visitor' under Appendix V. You can (and should) download and study the guidance. Canadians apply through the Visa Facilitation Centre and their applications are processed through the hub at the British Consulate General in New York (Canada was hubbed out a long time ago). It takes about 10 working days.

If I apply for a partner visa will I get refused?

No. Partners apply under a different set of rules wherein they expect you to live here. Different category, different semantics, different rules, different topology, different assumptions, etc, etc... There are tons of spouses living in the UK with the same 'X' stamp, as explained, it happens all the time.

How will having an X in my passport affect me?

Once you have successfully applied for entry clearance, they will put a nice shiney entry clearance in your passport that takes up a full page. It will mitigate the 'X' stamp. You will (or should) be able to explain that the 'X' stamp was the result of a judgement error (or whatever reason you think up). The Immigration Officer will always ask you about it, but when you have an entry clearance, you are 'pre-approved' to visit the UK. When an IO spots an entry clearance, a different procedure kicks which is much more of a formality because he/she knows that an ECO has already carefully reviewed all your stuff and "cleared" you.

Note: If you plan to visit Europe, you should use the same procedure to get a Schengen visa in your passport. They will worry about your removal until they see a proper visa in your passport.

Note: As a strategic tip, when you apply for entry clearance and you have to explain what happened to cause the removal, don't say it was the IO's fault. Say it was a judgement error or that you were tired and confused or whatever. They don't like it when people try to blame the IO because it means the person doesn't understand how the system works. It doesn't mean the IO is always right, but for applicants, having to explain the removal without blaming the IO is just part of 'what is'.

Note: Entry clearance is not mandatory for Canadians, even after a removal. You can always try to secure leave to enter at a UK port without it. You might get in or you might not. This answer is a 'best practices' approach; when there's multiple removals, the person faces the possibility of a ban.

Note: The American Embassy has an interesting article about getting bounced. About 99% of it is applicable to Canadians, so it's worth reading for background information.

Related article: UK entry denied: how to go about ensuring access next time

I'd like to add one thing to Gayot Fow's excellent answer: If you genuinely believe you were treated unfairly at the UK border (e.g. border agent ignored material facts, like the fact that your resume was 3 years old, or that you traveled many times without overstaying before), make sure to exhaust all your complaint/appeal options. Yes, the chances are probably slim that it will change anything, but it doesn't cost anything to file a complaint, and this is the only process as far as I know by which a decision at the border may be completely annulled.

I actually had to deal with a somewhat similar situation with the UK before, but it was a long time ago and the URLs I have from back then don't seem to work. A brief search seems to suggest that the new process is described in https://www.gov.uk/government/organisations/uk-visas-and-immigration/about/complaints-procedure - but I'm not 100% sure this is the correct process, I only did a quick search.

Also, keep in mind you can do an FOI (Freedom of Information) request to try and get access to the notes that the officer has entered about your case. These requests take a long time, so file early if you want to get the results before filing your complaint (and don't delay filing the complaint, it says 3 months but from what I saw in my case, the earlier you file the higher the chances of success).

One more thing: If you're treated unfairly at the UK border, and see that a bad decision is inevitable, don't be afraid to ask for the border agent's supervisor. (Actually, in the case I was appealing, the response was basically that I should have asked for a supervisor on the spot).