Cheap, abundant U.S. shale gas soon will find its way to a new market: The flood of people moving from the countryside to the dense cities of developing countries. In a few years, the gas will arrive in China, India and Latin America as resin pellets that will become plastic products like food packaging, a key component for urbanizing populations as consumers trade farming markets for preserved foods. It’s a huge, hungry market. Shortly after U.S. gas prices collapsed a few years ago, a half-dozen energy companies raced to build plants along the Gulf Coast that can break natural gas...