Synthesis gas is produced from a methane-containing reactant gas in a mixed conducting membrane reactor in which the reactor is operated to maintain the product gas outlet temperature above the reactant gas feed temperature wherein the total gas pressure on the oxidant side of the membrane is less than the total gas pressure on the reactant side of the membrane. Preferably, the reactant gas feed temperature is below a maximum threshold temperature of about 1400.degree. F. (760.degree. C.), and typically is between about 950.degree. F. (510.degree. C.) and about 1400.degree. F. (760.degree. C.). The maximum temperature on the reactant side of the membrane reactor is greater than about 1500.degree. F. (815.degree. C.).