a system and method for gasification of a feedstock in a subterranean formation to produce syngas is described . an injection well is completed in the formation to inject an oxidant , provide an ignition source and convey the feedstock that includes water and one or more of a biomass , waste plastic , coal , bitumen and petcoke . volatized hydrocarbons and gaseous reaction products are simultaneously withdrawn from a producer well from the subterranean formation to the surface . this syngas product is treated at the surface for power generation or conversion to transportation fuels and / or plastics . this method provides a low capital cost gasification unit which is capable of processing a variety of feedstock mixtures .