A 1-dimensional heterogeneous reactor model with the gas-solid interfacial phase gradients wasdeveloped for the simulation of the packed bed reactor where the exothermic reversible water gas shift reactionfor the natural gas steam reformed gas was proceeding in adiabatic mode. Experimental results obtained over theWGS catalyst, C18-HA, were best simulated when the frequency factor of the reaction rate constant was adjustedto a half the value reported over another WGS catalyst, EX-2248, having the same kinds of active componentsas the C18-HA. For the reactor of the inside diameter 158.4 mm and the bed length 650 mm, the optimum feedingtemperature of the reformed gas was simulated to be 194°C, giving the lowest CO content in the product gasby 1.68 mol% on the basis of dried gas. For reactors more extended in the bed length, the possible lowest COcontent in the product gas with the optimum feeding temperature of the reformed gas were suggested.