That's slightly different. Jon_in_london wants to use the heat to create more useful energy, not use it as heat itself.
So to be quick about it: the cost to put in giant engines to try and use that heat would be more than the cost of the energy you would expect to extract. For people who like thermodynamics, I'll try to explain in more detail.
A closed, Rankine-cycle (standard water/water-vapour cycle) system generates power by compressing liquid water, heating that into steam, decompressing the steam through a turbine (which reduces the temperature of the steam), and cooling the steam into a liquid so it can be pumped.
So you ask, "why not use the turbine so it cools the steam into water? It is, physically, possible." The problem is that you then have flowing steam condensing into water on your precious, extremely carefully designed turbine. This will wreck your turbine blades very quickly. You do what you can to get the steam to not condense (use very low pressure, so the water's boiling point is lower), but ultimately it's still a bit hot coming out, and there isn't a good way to adapt you high temperature system to use the energy in the steam as it condenses to water (the temperature is actually constant for most of the cooling phase, like how the temperature of boiling water is constant).
So let's build a second engine to use that energy, right?
In general, any heat engine (thing that turns high temperatures into mechanical/electrical energy) is limited by the Carnot efficiency. This efficiency is
[latex]\eta = 1 - \frac{T_c}{T_h} [/latex]
T is the temperature in Kelvin, of your "cold" temperature (where you extract heat) and your hot temperature (where you add heat).
So you want a high starting T and a low ending T. The basis of any closed heat engine is being able to put in heat at high pressure and take out heat at low pressure, so you need to operate at a temperature above ambient, which is going to be up to 300K, but for cooling water could be more like 290K most of the time.
But remember about how this waste heat is really low pressure steam? It can be as low as 40 Celcius, which is only 313 K. Your cycle will only have, at absolute best, 7% efficiency. 2% would be much more realistic, because it has to be above the cooling water temperature.
Bear in mind the power plant can have 40% or greater efficiency. The 'waste' heat is only 60% of what the first cycle(s) worked with. So 2% of that 60% gives more like 1% improvement, for an addition that would have a price comparable to the original system.
