A method and an apparatus for gasifying carbonaceous material, in which (a) gasifying carbonaceous material is gasified in a gasification reactor of a gasification system to produce a product gas, (b) the product gas, ash particles entrained with the product gas, residual carbon, and gasified tar compounds are discharged from the gasification reactor to a product gas channel, (c) the product gas discharged from the gasification reactor is cooled using a gas cooler disposed along the product gas channel, so that the tar compounds are condensed to a liquid from that tends to stick to heat exchange surfaces of the gas cooler, (d) solid material including the ash particles and the residual carbon is separated from the gasification system, (e) the solid material separated from the gasification system is guided to an ash reactor, and oxygen-containing gas is supplied to the ash reactor, whereby the residual carbon in the solid material reacts with oxygen, and additional ash particles and exhaust gas are generated, and (f) ash particles from the ash reactor are guided along a conveying duct to the gas cooler or to a location upstream of the gas cooler, thereby increasing the ash content of the product gas and decreasing the tendency of the condensed tar compounds to stick to the heat exchange surfaces of the gas cooler.