Any furnace with an AFUE rating of 90% or higher is considered a condensing furnace.  That is because so much of the heat is removed that the exhaust gases actually cool and condense in the vent pipe.  That means that the air going up the vent cools down to the point where it meets dew point.  Therefore it begins to drip water back down the exhaust pipe.

This type of furnace always vented with PVC instead of metal pipe.  The furnace has a drain that allows the water drain out of the bottom.