Life-cycle energy balances and emissions of CO, and other gases have been evaluated for the following systems: an LNG combined-cycle(LNGC/C), an integrated coal-gasification combined cycle (IGCC) and a molten carbonate fuel cell (MCFC) combined-cycle power-generation systems with CO2 capture and sequestration technologies. The predicted CO2 recovery and sequestration will lower the net energy ratio (which measures the total efficiency of a fossil-fuel system) by 16 to 38% for the LNG C/C system, by 21 to 57% for the IGCC system and by 17 to 56% for the MCFC system. The CO2-emission control potentials for the LNG C/C, IGCC and MCFC are in the range 64-72%, 65-76% and 57-68%, respectively. However, off-site power required for CO2 emission control increases NOx and SOx emissions by as much as a factor of 1.3 to 10.