In this work, tri-reforming and steam reforming processes have been coupled thermally together in a reactor for production of two types of synthesis gases. A multitubular reactor with 184 two-concentric-tubes has been proposed for coupling reactions of tri-reforming and steam reforming of methane. Tri-reforming reactions occur in outer tube side of the two-concentric-tube reactor and generate the needed energy for inner tube side, where steam reforming process is taking place. The cocurrent mode is investigated, and the simulation results of steam reforming side of the reactor are compared with corresponding predictions for thermally coupled steam reformer and also conventional fixed-bed steam reformer reactor operated at the same feed conditions. This reactor produces two types of syngas with different H-2/CO ratios. Results revealed that H-2/CO ratio at the output of steam and tri-reforming sides reached to 1.1 and 9.2, respectively. In this configuration, steam reforming reaction is proceeded by excess generated heat from tri-reforming reaction instead of huge fired-furnace in conventional steam reformer. Elimination of a low performance fired-furnace and replacing it with a high performance reactor causes a reduction in full consumption with production of a new type of synthesis gas. The reactor performance is analyzed on the basis of methane conversion and hydrogen yield in both sides and is investigated numerically for various inlet temperature and molar flow rate of tri-reforming side. A mathematical heterogeneous model is used to simulate both sides of the reactor. The optimum operating parameters for tri-reforming side in thermally coupled tri-reformer and steam reformer reactor are methane feed rate and temperature equal to 9264.4 kmol h(-1) and 1100 K, respectively. By increasing the feed flow rate of tri-reforming side from 28,120 to 140,600 kmol h(-1), methane conversion and H-2 yield at the output of steam reforming side enhanced about 63.4% and 55.2%, respectively. Also by increasing the inlet temperature of tri-reforming side from 900 to 1300 K, CH4 conversion and H-2 yield at the output of steam reforming side enhanced about 82.5% and 71.5%, respectively. The results showed that methane conversion at the output of steam and tri-reforming sides reached to 26.5% and 94%, respectively with the feed temperature of 1100K of tri-reforming side. Copyright (C) 2013 John Wiley & Sons, Ltd.