Multi-objective optimisation methodology is ideally suited to incorporate environmental objectives in the optimal design of energy conversion systems. It is also desirable to optimise life-cycle emissions rather than local emissions only. However, using LCA results directly as an objective function gives sub-optimal designs, since inputs whose life-cycle emissions are easy to reduce are underused and vice-versa. We develop a method integrating LCA results in an optimisation framework, so that the optimal designs found are logically consistent with the aim of minimising global emissions. We then apply the method to the optimisation of a NGCC power plant with CO2 capture, optimising column dimensions, solvent flow and heat exchange configuration with respect to two functions: cost and life cycle global warming potential. This method of using LCA results has an actual benefit since the cost of reducing local emissions cannot be lower than for global emissions.