Process for producing synthesis gas

A process for producing hydrocarbons, in which a first feed substream and a second feed substream are obtained from a hydrocarbonaceous feed stream, of which the first feed substream is converted by means of partial oxidation or autothermal reforming to a first synthesis gas stream and the second feed substream is converted by means of steam reforming to a second synthesis gas stream and subsequently combined with the first synthesis gas stream to give a third synthesis gas stream, of which at least a first portion of the third synthesis gas stream is converted by Fischer-Tropsch synthesis to a crude product stream comprising hydrocarbons of different chain lengths, from which light hydrocarbons are separated in a tail gas, in order to recycle them and use them in the partial oxidation or autothermal reforming, characterized in that unsaturated hydrocarbons are separated from at least a portion of the tail gas by a cryogenic separation method in order to obtain a stream which is substantially free of unsaturated hydrocarbons as a feed for the partial oxidation or autothermal reforming.