Modern condensing gas boilers for domestic use are better than 90% efficient, the exhaust gas is very cool so I don't think that thermoelectric devices have much to offer there.
There are systems here listed at 98.5 AFUE
However in real boilers, there is no need for this - by condensing that water vapor back into liquid water you can get more energy out. That's the 'Higher heating value'.
When measuring efficiency, we always use the lower heating value, whereas we ought to use the higher heating value. Therefore a "98% efficient furnace" is actually more like 88% efficient when considering the higher heating value.
Also, cold exhaust doesn't exhaust (since it cant rise through a density change). So, if your exhaust is too cold, it will collect in your basement (CO2 should be a bit denser than air, so absent any drafts it'll collect)
The previous owners of my house installed my high efficiency stove improperly using the old brick flue. The fumes eventually ate away at the mortar and a few bricks fell I'm clogging it.
Unless you have a fan blowing smoke into flue. Without fan, my furnace would not work, exhaust is only slightly above water temperature and furnace is designed so that condensate pools in special container, which you have to empty or connect to sewage.
The place this would be useful is in capturing otherwise unusable energy from low grade heat waste. For example, the output water of a steam turbine might still be "hot", but there's very little you can do with it - it's in the wrong place for heating houses, it's too cold for steam turbines. Generally that energy is lost to the air.