An inventive adsorptive gas separation process is provided capable of producing a purified methane product gas as a light non-adsorbed product gas as opposed to a heavy desorbed exhaust gas component, from a feed gas mixture comprising at least methane, and carbon dioxide. In an embodiment of the invention, the feed gas mixture may comprise at least about 10% carbon dioxide, and the purified methane product gas may be desirably purified to contain less than about 5000 ppm carbon dioxide. In another embodiment of the invention, the feed gas mixture may comprise at least about 50% carbon dioxide, and the purified methane product gas may be desirably purified to contain less than about 5000 ppm carbon dioxide.