The behavior of high temperature fuel cells fed by urban solid waste biogas, consisting mainly of methane and carbon dioxide, is investigated by using a properly made model. For these types of fuel cells, an indirect internal reforming is implemented, which, although less efficient in terms of heat exchange, is more appropriate with regard to the problem of carbon deposits. Therefore, special attention is focused on the presence of carbon dioxide that causes the lowering of hydrogen production and possible problems of carbon deposits, thus, affecting the conversion efficiency. A zero-dimensional numerical simulation model is formulated and subsequently implemented in a Matlab computing environment.