The disclosed invention relates to a process and apparatus for converting natural gas to higher molecular weight hydrocarbons. The process includes steam reforming to form synthesis gas followed by a Fischer-Tropsch reaction to convert the synthesis gas to the high molecular weight hydrocarbons. The reforming and Fischer-Tropsch reactions are conducted in microchannel reactors. The higher molecular weight hydrocarbons may be further treated to form hydrocarbon products such as middle distillate fuels, lubricating oils, and the like. The apparatus includes vessels containing SMR microchannel reactors and Fischer-Tropsch microchannel reactors. A composition comprising a mixture of olefins and paraffins is disclosed.