We've done exactly what you describe -- my husband's daily commuter had a double up front, and he just removed the big chain ring, took off the rear cassette and replaced it with a freewheel (he has horizontal dropouts so no need for a chain tensioner), adjusted the chain length, and voila. But his chain line isn't perfect and it doesn't always work out so easily. Usually you have to re-dish the rear wheel in order to make the chain line straight.

You could also replace the rear wheel with a track wheel and that would take care of the problem. This is what I've done on my singlespeed conversions.