They use it in the city heating system, which is obviously the optimal way of reusing the heat but most data centers aren't put next to such a heating system.

They use it in the city heating system, which is obviously the optimal way of reusing the heat but most data centers aren't put next to such a heating system. I'm talking about making electricity back from the heat (using a low-temp thermodynamic cycle). It has a low yield (due to the low input temperature) but it's usually economically viable when using heat that would end up in the heavens anyway.