Pep wrote:
I have had two contractors in to price out a new high efficiency (90%)
gas furnace ...

One didn't recommend the high efficiency furnace. He said that the new
exhaust will produce a sizable condensation cloud which will frost over
a window which is about 3-4 feet above the exhaust. As well he thought
condensation would freeze on my neighbors house which is about 6-8 feet
away.

Does a high efficiency gas furnace really produce a cloud of
condensation? (It gets quite cold here, with a lot of freeze thaw
cycles over the winter)

Yes it does. However...

Would the furnace exhaust come out with enough force to frost up my
neighbor's exterior wall 6 feet away?

We have a 90+% efficient furnace and while there is a bit of a cloud it
would not frost up a window 6 feet away. I suppoe the ones they
recommend could be different, though.