The primary steam reformer (PSR) is the heart of Ammonia unite in Basra Fertilizer plant. It consists of catalyst tubes that located at the center of side-sired furnace. Steam reforming process of natural gas based on endothermic reactions is carried out in fixed bed reactors produce synthesis gas (CO and H2). Three important limitations that affect the performance of PSR; thermodynamic equilibrium of reforming reactions, heat and mass transfer resistance and carbon formation. Two models are used in this study to simulate the PSR; radiative model for sidefired furnace and first order heterogeneous model or fixed bed reactor taking in account the above limitations. The simulation results is compared with operating data that carried on Basra fertilizer plant and they match well. The simulation program is used to discuss the effect of more important operating conditions; temperature, pressure and steam to methane ratio (SCR) of the feed, on composition of synthesis gas, reactor wall temperature, pressure drop and heat duty.