No. There are several reasons not to attempt heating a greenhouse with a high-efficiency furnace vent:
1) Carbon monoxide could be lethal to anyone entering a greenhouse heated by furnace exhaust.
2) Carbon monoxide interferes with photosynthesis in plants.
3) It would not be worth the investment.
You have already been environmentally responsible by investing in a high-efficiency furnace. You could run the exhaust pipe through a greenhouse, but there should be very little heat in the exhausted air from your high-efficiency furnace. Feel the plastic exhaust vent — it should not be much warmer than room temperature. The small amount of residual heat helps move the exhaust fumes up the flue and out of your house. If the exhaust pipe is too long or the exhausted air too cool, the furnace may not vent properly, and carbon monoxide could seep back into your home from the furnace. The steamy exhaust you see is due to the moisture (water) that forms as a breakdown product of combustion. The presence of steamy exhaust does not mean a lot of heat energy is being lost — you can also see your breath on cold days.
Carbon monoxide is just too dangerous, and your furnace has already extracted most of the heat out of the exhausted air anyway.
For more information, see: Combat Higher Fuel Prices with Efficient Heating Systems and Fuels and Alternate Heat Sources for Commercial Greenhouses.