Condensation build up by furnace?

We have a 4 yr old high efficiency gas-fired furnace in our attic. It's been giving us trouble EVERY YEAR since the house has been built. This year is a new issue- we noticed water on our ceiling so went into the attic and found water collecting at the base of the trunk that comes off the furnace. Does anyone know what the heck is going on or how to fix it? My husband is going crazy! Of course the heating/plumbing guys are booked solid for several days since it's only 15 degrees and we just got a foot and a half of snow. Thank god we have a propane fireplace for supplemental heating, but we're worried about the damage done to the attic with all the water. HELP!