One-step methanation tubular cell integrating high temperature Solid Oxide Electrolysis Cells (SOEC section) at 800 degrees C and low temperature Fischer-Tropsch-like reactor (FT section) at 250 degrees C is a novel design for energy conversion from power to fuel. In this simple and compact design, syngas (H-2/CO mixture) produced from co-electrolysis of H2O/CO2 inside SOEC section can undergo reversed methane reforming reaction at favorable low temperature in the FT section for CH4 production. A 2D-axisymmetry model is developed to study the coupled transport and reactions in the methanation cell. The heat and mass transfer, electrochemical reactions and chemical reactions such as Waster Gas Shift Reaction (WGSR) and Methane Steam Reforming Reaction (MSR) are fully considered. Parametric simulations are conducted to investigate the effects of applied voltage, length of FT section, operating temperature and inlet gas composition on CH4 generation. Optimal inlet gas composition is found both with (H-2: CO2 = 3.566, 20% vol H2O) or without (CO2: H2O = 0.3566) H-2 assisting. (C) 2016 The Electrochemical Society. All rights reserved.