Here's how the "efficiency" works: natural gas is burned in a turbine (similar to a jet turbine). The turbine generates electricity and sends out extremely hot fumes. The hot fumes are sent to water boilers which produce steam. The steam turbines produce more electricity. Hot water from the boilers are cycled underground through out downtown to provide hot water (sometimes steam is cycled too). Hot water that not used, is cooled (by the air) in giant cooling towers. By the time the water gets to the cooling towers it's usually lukewarm. What makes it efficient is the fact that the system tries to squeeze as much heat out of the water as it can to produce electricity. When we can't produce anymore electricity, we use it's how water in our buildings. Note that electricity generation is done twice: with gas turbine and a steam turbine. The electricity can be used to power the buildings, and at times, be sold on the power grid for additional revenue. A lot of universities also do this (including the UC Davis Medical Campus in Sacramento).