Natural gas is falling out of favor with emissions-wary investors and utilities at a quicker pace than coal did, catching some power generators unaware and potentially leaving them stuck with billions of dollars of assets they can’t sell. Citigroup Inc. and JPMorgan Chase & Co. are among the banks that strengthened their financing restrictions on thermal coal under pressure from shareholders wanting to avoid the fuel, and the expectation is that gas is next. Executives at some western European companies say they’re already struggling to sell gas-fired facilities.