In order to produce synthesis gas (syngas), four reforming processes including two stand-alone primary and secondary reformers and two combined configurations are investigated. With changing operating parameters and arrangement of the reformers (i.e. stand-alone, parallel, and series), the syngas may be obtained for different applications such as methanol, Fischer-Tropsch (FT), and ammonia synthesis. After study of several arrangements, the selected cases are simulated. Due to the shift some reforming duty from the primary reformer to the secondary one, the primary size and the total fired duty are reduced. The Non-Sorting Genetic Algorithm II (NSGA-II) optimization method is applied for the problem based on the practical point of view. It is shown that the parallel case is preferable in accordance to the high productivity object. For an optimum point, parallel case has 58% of productivity of syngas more than that of series one. However, the series configuration consumes lower fuel (361.1 kmol h(-1), in comparison to the parallel case with 437.19) and releases lower amount of CO2 emission. It is shown that the series arrangement has an average of 13.22% of released CO2 molar flow less than that of parallel arrangement. Crown Copyright (C) 2010 Published by Elsevier B.V. on behalf of The Institution of Chemical Engineers. All rights reserved.