Methane reforming with CO2 has been intensively investigated for solar thermochemical energy storage, including endothermic reactor design, reaction kinetics study and endothermic reactor performance analysis. A considerable body of research has been amassed concerning the charging loop, i.e., solar CO2 reforming of methane. However, little research has been conducted on the discharging loop, i.e., the subsequent heat recovery in a methanation reactor at temperatures high enough for electricity generation. This paper explores the preliminary design of a methanation reactor for producing sCO(2) at 700 degrees C in solar thermochemical energy storage. A two-dimensional, steady-state, pseudo-homogeneous model is proposed to simulate heating secondary working fluid, e.g., sCO(2), with methanation reaction in a tubular packed bed reactor. The model is validated by comparing temperature profiles and methane conversions generated in this study with the established data from other publications. Parametric studies are performed to investigate the effects of reactor diameters and mass flow rates on the reactor capital cost and energy efficiency. The results show that the estimated catalyst cost is comparable to the reactor wall material cost although the unit price of the catalyst is much higher. Basically, enhancing heat transfer by decreasing reactor diameters can reduce the capital cost. But the energy efficiency decreases with reactor diameters decreasing due to a larger pumping power required. Similarly, enhancing heat transfer by increasing the mass flow rates, i.e., the gas mass flow rate and the sCO(2) mass flow rate, also reduces the cost. Different from the effect of decreasing reactor diameters, increasing the sCO(2) mass flow rate cannot only reduce the cost but improve the energy efficiency, while increasing the syngas mass flow rate leads to a lower energy efficiency.