The invention relates to a method for the gasification of carbonaceous materials in the form of solid particles. The inventive method comprises the following steps consisting in: heating a gaseous mixture formed by a feed gas and water vapor in a heater (1); bringing the particles of the carbonaceous materials into contact with the heated gaseous mixture in a pressurized gasification reactor (2), with the formation of a raw reaction gas and unburnt ash; separately supplying the heater with (i) water vapour and (ii) the aforementioned feed gas; taking separate samples from the heater of the gaseous mixture (at point 13) and of part of the feed gas in the dry state (at point 16); and injecting into the gasification reactor said dry feed gas and a gas forming therewith a fuel mixture in the ash.