The design and optimization of a natural gas to hydrocarbon liquids (GTL) process is considered, mainly from the view of maximizing the variable income during operation. Autothermal reforming (ATR) is used for synthesis gas production. The kinetic model for Fischer-Tropsch (FT) reactions is the one given by Iglesia et al, for a cobalt-based FT reactor. For the product distribution, three alternative expressions for the chain growth factor a are compared.