They it buys a turbine and fuels it with natural gas. Capital costs are minimal, and regulators can't easily stop you from passing fuel costs on to the consumer. It turns out the negawatters are wrong on both counts. First, natural gas is largely methane, which is a twentyfold more powerful greenhouse gas than carbon dioxide. And the gas distribution system leaks.
Nobody really knows.
Long-distance pipelines are tight, but urban distribution systems aren't. Methane, unlike carbon dioxide, breaks down in the atmosphere after a decade or two but still has plenty of time to warm things up. Second, and most embarrassing to the antinuke crowd, gas contains traces of radon. But when gas is burned in the home, as a substitute for electric heat perhaps, the radiation doses are higher. Still tiny, except by comparison with better alternatives.