Can I use the exhausted heat from a high-efficiency furnace vent to heat a greenhouse?

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 …