As there seems to be some confusion about the different terms, I try to make myself more clear:
You have your CP interview in Montreal. They will keep your passport to put the IMMIGRANT VISA in it. Since you cannot cross the border without your passport, you have to wait till you get the visa).
Once you go back to the US, you activate your greencard with your immigrant visa and the sealed envelope at the border/port of entry. From this point on, the immigrant visa in your passport serves as a TEMPORARY I-551 (="temporary greencard"), you are now a lawful permanent resident of the US.
After a couple weeks (sometimes longer), you will receive the plastic GREENCARD by mail.