Abstract Future energy supplies are uncertain, especially in view of environmental problems, fuel prices, and energy demand. In this paper two types of novel energy plants and a novel energy system have been proposed to cope with these conditions. The first type can utilize multiple kinds of fossil fuels and economically remove CO 2 and the second can produce multiple kinds of final energy to cope with fluctuations in the energy demand. Characteristics of a novel energy system (a Flexibly Integrated Energy System; FIES), which includes novel energy plants as described in the foregoing, have been investigated with a linear programming model. A reference system has also been represented which behaves similarly to the present energy system for comparison. Computed results have demonstrated that CO 2 emissions can be reduced more economically and efficiently than in the reference system if it is combined with CO 2 removal systems.