Integrated Gasification Combined Cycle plants (IGCC) are efficient power generation systems with low pollutants emissions. Moreover, the entrained flow gasifier of IGCC plants allows the combined use of other lower cost fuels (biomass and waste) together with coal. Co-firing with biomass is beneficial for the reduction of CO2 emissions of fossil source. In this paper the results of co-gasification tests with two types of biomass deriving from agricultural residues, namely 2% and 4% by weight of olive husk and grape seed meal, in the 335 MWeISO IGCC power plant of ELCOGAS in Puertollano (Spain) are reported. No significant change in the composition of both the raw syngas and the clean syngas was observed. Furthermore, a process simulation model of the IGCC plant of Puertollano was developed and validated with available industrial data. The model was used to assess the technical and economic feasibility of the process co-fired with higher biomass contents up to 60% by weight. The results indicate that a 54% decrease of fossil CO2 emissions implies an energy penalty (a loss of net power) of about 20% while does not cause significant change of the net efficiency of the plant. The mitigation cost (the additional cost of electricity per avoided ton of CO2) is significantly dependent on the price of the biomass cost compared to the price of the fossil fuel. (C) 2013 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.