Abstract: Hydrocarbon feedstocks are converted into synthesis gas in a two-stage process comprising an initial steam reforming step followed by final conversion to synthesis gas in a mixed conducting membrane reactor. The steam reforming step converts a portion of the methane into synthesis gas and converts essentially all of the hydrocarbons heavier than methane into methane, hydrogen, and carbon oxides. The steam reforming step produces an intermediate feed stream containing methane, hydrogen, carbon oxides, and steam which can be processed without operating problems in a mixed conducting membrane reactor. The steam reforming and mixed conducting membrane reactors can be heat-integrated for maximum operating efficiency and produce synthesis gas with compositions suitable for a variety of final products.