For those who don't know what co-generation is, a combustor burns natural gas and spins the turbine (similar in principle to an jet engine) which is connected to a generator which produces electricity. This system is traditionally unefficient on it's own, since much of the energy is used to compress air to feed the combustor.
By sending the hot exhaust into a boiler setting, steam can be produced, which in turn is used to turn a traditional steam turbine which is also connected to a generator which produces electricity.
Coupled, the two generators are able to reach efficiencies much higher than coal fired plants. Additionally, natural gas is mostly pollution free, except for some NO, and NO2, which is minimized by the design of the system.
Bayside power consists of this type of system, but is much smaller than 1500MW.
Source: Pan Global Systems