This work aims to develop a stationary phenomenological model of an industrial steam reforming unit. Unlike the usual approach in the literature, the combustion reactions at the furnace are described through rigorous distributed mass and energy balances to predict the concentration and temperature profiles along the length of the equipment. A more precise prediction of these profiles is useful for understanding and monitoring the quality of the outlet gas for different inlet conditions without the need to know the length of the flame. This model further considers the refractory as an additional control volume, allowing us to predict and avoid temperature gradients that could damage the equipment and lead to operational downtime. The heat transfer effects are described in detail as the radiation reabsorbed by a different element inside the same phase (i.e., two tubes or refractory walls exchanging radiant energy), the radiant heat absorbed by the gas inside the furnace, and the heat transferred by radiation inside the tubes. The model is validated against different case studies using industrial and literature data. The model predicts reformed and flue gas compositions with maximum relative deviations (with respect to experimental data) of 3.27% (case 1) and 11.14% (case 5), respectively, proving the adequacy of the proposed model. Sensitivity analysis is performed to investigate the influence of some heat transfer phenomena, often neglected in the literature, on the performance of the developed model. Through this analysis, it was possible to identify the most important heat transfer phenomena (the radiant energy exchanged intraphase, i.e., between two tubes, and between phases, i.e., the tube and the process/furnace gases) and those that are negligible (convective heat transfer at the furnace). The proposed model might be useful for process monitoring as well as optimization.