This paper presents an integrated multi-level model of a solid oxide fuel cell system, which accounts for the effects of concentration, activation and ohmic polarizations on single-cell performance, as well as the cell-to-cell interactions in a cell stack module. Furthermore, this model extends the work of Lu and Mahoney (1988) and Harvey and Richter (1994) by including the performance of a cell stack operating with a fuel reformer, heat exchangers, and a steam generator over a range of design parameters. This paper also demonstrates the procedure by which a single-cell model is scaled to a system model.