The present invention relates to devices for conducting, simultaneously, exothermic and endothermic chemical conversions with transfer of heat therebetween. More particularly, this invention relates to autothermic modules using oxygen ion-conducting dense ceramic membranes to separate, selectively, oxygen from an oxygen-containing gas and supply it directly to partial combustion of gaseous organic compounds. Processes using autothermic modules in accordance with this invention are, advantageously, used for production of synthesis gas comprising carbon monoxide and molecular hydrogen which synthesis gas is substantially free of deleterious and/or inert gaseous diluents such as nitrogen. In particular, for conversions, within the integral autothermic module, of natural gas or other forms of gaseous lower alkanes to synthesis gas by means of partial combustion followed by reforming.