An integrated anaerobic digester-solid oxide and molten carbonate fuel cells system in co-generative arrangement is considered for a more rational use of energy. High temperature fuel cells are appropriate energy converters of biogas, especially molten carbonate fuel cells that enable the active use of the carbon dioxide by using it at the cathode section. The integrated system energy performances are investigated in relation to the amount of carbon dioxide fed to the anodes of the fuel cells. An energetic analysis, considering some important typical parameters of cogeneration plants, is carried out to validate the importance of the system in the combined heat and power set up.