This paper presents the steady state and transient model of a natural gas fuel processing system of a solid oxide fuel cell (SOFC) hybrid system, and its validation using data obtained through the use of a real plant. The model was developed by the Thermochemical Power Group of the University of Genoa, Italy, using the in-house tool TRANSEO working in the Matlab/Simulink environment, whereas the real plant was designed and built by Rolls-Royce Fuel Cell Systems Limited (RRFCS) to feed a 250 kWe SOFC hybrid system with a methane stream undergoing requirements about composition, pressure, and temperature. The paper presents in detail the fuel processing system and, with particular emphasis, the selective catalytic sulphur oxidation (SCSO) and the catalytic partial oxidation (CPOx) subsystems. Thanks to the collaboration between the University and RRFCS, in the model the real physical properties of the different materials and geometry of the components have been carefully used. The transient model has been fully validated against experimental data obtained from long duration tests, which included the warm-up, part and full load operation, and cool-down phases of the external fuel processing system. In the validation process both gas and wall temperatures have been taken into account. The transient model has shown the ability to predict satisfactorily the plant behavior both at steady-state and transient conditions. The validated model is now under further development to be used for dynamic control system applications.