The invention provides an improvement in the utilization of hydrocarbon feedstock by partial oxidation with oxygen to form a synthesis gas comprising carbon monoxide and hydrogen and subjecting the synthesis gas to a conversion process comprising an exothermic reaction. The oxygen is provided by air separation in which the feed air is at least partially compressed by work generated by expansion of a working fluid vaporized by indirect heat exchange with at least one of the synthesis gas and the exothermic reaction. The improvement is that the working fluid is preheated by indirect heat exchange with adiabatically compressed feed air, thereby improving the overall efficiency of the process and reducing capital costs compared with conventional generally isothermal feed air compression. Preferably, the gas conversion process is a catalytic hydrogenation to prepare paraffinic hydrocarbons (Fischer-Tropsch reaction), methanol or dimethylether.