Co-gasification of coal and biomass in an existing coal-fired IGCC power plant is proposed as an efficient, flexible and environmentally friendly way to increase the biomass contribution to electricity generation. A model of an entrained flow gasifier is described and validated with nearly 3,000 actual steady-state operational data points (4,800 hours). The model is then used to study co-gasification of coal, petroleum coke and up to 10 percent of several types of biomass. As a result, the influence of fuel variations on gasifier performance and modifications in operation that should be made in co-gasification are obtained. A conclusion of our study is that co-gasification is possible provided that operation is properly adapted. A validated model can be very useful for predicting operating points for new fuel mixtures.