A process for producing hydrocarbons comprises providing a multi-tubular reactor having at least 100 tubes units containing a catalyst, each tube being between 2 and 5 meters tall and in thermal contact with a cooling fluid; feeding hydrogen and carbon monoxide to each tube at a linear gas superficial velocity less than about 60 cm/s; and converting the gas feedstream to hydrocarbons on the catalyst, wherein the yield of hydrocarbons in each tube is greater than 100 (kg hydrocarbons)/hr/(m3 reaction zone). Each tube may have an internal diameter greater than 2 centimeters. The catalyst may be active for Fischer Tropsch synthesis and may comprise cobalt or iron. The maximum difference in the radially-averaged temperature between two points that are axially spaced along the reactor is less than 15° C., preferably less than 10° C. The catalyst loading or intrinsic activity may vary along the length of the reactor.