How best to remove fumes after applying polyurethane in a hardwood refinishing project?
Turn up the heat while opening windows across the house.  Put in fans in windows/doors blowing in the house on one or two sides.  Just don't put fans blowing in all windows so air has an easier chance of escaping.  It would be better to open all windows partially than two windows fully.

Definitely make sure you have at least one fan though to create a pressure differential to push out the inside air.  As an example, we have used 6 box fans with filtered air on the intake fans sealed to an opening while lacquering cabinets (extremely high fumes) and it kept the fumes down to a minimum throughout the rest of the house DURING spraying.  You have less fumes so even a couple fans could do a world of difference.

Consider the heat a cost of doing the project.  Might cost $100-$200 in heat over 5 days if you really opened things up but should be less.  Just depends on how long, heating method, fuel cost, and how much you change out the air.

You could also purge the house at intervals by doing this for 1-2+ hours or so at a time 1+ time a day until you are satisfied with the fume level.

You might need to re-purge even weeks later if your house is really sealed up and you are concerned about the fumes.