The implementation of integrated energy systems within urban areas is a promising CO?2 emissions abatement measure. In this paper on-site heating, cooling and power generation based on a solid oxide fuel cell and gas turbine (SOFC-GT) combined cycle unit associated with a compression chiller and additional boilers has been considered from the viewpoints of cost and CO?2 emissions. Physical and costing modelling of such a unit has been integrated withn a new multi-criteria evolutionary algorithm for an assessment of the economic and environmental performances associated with optimal design and operation, for typical requirements of large office buildings in Tokyo.