An experimental study of entrained flow, air-blown cogasification of biomass and a coal−coke mixture has been performed in order to evaluate the effect of the relative fuel/air ratio (ranging between 2.5 and 7.5), the reaction temperature (ranging between 750 and 1150 °C), and the biomass content in the fuel blend on the producer gas composition and the process performance. Dealcoholized grape marc, a waste coming from the wine industry and with high potential in the south-central regions of Spain, has been chosen as biomass fuel. On the other hand, the coal−coke blend (composed of a low-rank autochthonous coal and a solid residue from refineries) is an abundant fossil fuel which is used in a commercial IGCC power plant. The results obtained show that an increase of the biomass content in the fuel blend upgrades the producer gas quality and improves the cold gas efficiency. Some hints of synergy between biomass and coal−coke have been found, especially at low fuel/air ratios and low reaction temperatures, which might be mainly related to the content and composition of the blend ash (especially due to the catalytic effect of Ca and K coming from the biomass ash, and the Fe, Ni, and Zn contents of the coal−coke ash). However, thermogravimetric analyses have not provided enough information about possible interaction between biomass and coal−coke.