A method of producing a gas from a combustible material (3, 220, 320, 420) by the injection of an oxidant, the method comprising the steps of: (a) loading the combustible material (3, 220, 320, 420) into a containment structure (33, 110, 210, 310, 410); (b) substantially sealing the containment structure (33, 110, 210, 310, 410); (c) feeding an oxidant into the sealed containment structure (33, 110, 210, 310, 410) and igniting the combustible material (3, 220, 320, 420) to form a gasification zone (323, 324, 325, 423, 424, 425); (d) moving an oxidant injection point from one end to the other end along a horizontal axis at the base of the structure (33, 110, 210, 310, 410) and injecting the oxidant thereby to contact the combustible material (3, 220, 320, 420) at multiple points in a sequence; (e) controlling the product gas flow rate and composition by varying a flow rate of oxidant and a rate of movement of the oxidant injection point such that the combustible material (3, 220, 320, 420) is partly converted, thereby leaving behind an unconverted, thermally affected layer of combustible material (3, 220, 320, 420) in the sealed containment structure (33, 110, 210, 310, 410) after the injection point has passed through the material, wherein the conversion of the combustible material (3, 220, 320, 420) to a gas at one point in the sequence is initiated prior to complete conversion of the combustible material (3, 220, 320, 420) at a previous point in the sequence; f) cooling and purging the sealed containment structure (33, 110, 210, 310, 410) and loading fresh combustible material (3, 220, 320, 420) on top of the thermally affected layer left behind after completing steps c) to e); and g) repeating Steps b) to f). (a) loading the combustible material (3, 220, 320, 420) into a containment structure (33, 110, 210, 310, 410); (b) substantially sealing the containment structure (33, 110, 210, 310, 410); (c) feeding an oxidant into the sealed containment structure (33, 110, 210, 310, 410) and igniting the combustible material (3, 220, 320, 420) to form a gasification zone (323, 324, 325, 423, 424, 425); (d) moving an oxidant injection point from one end to the other end along a horizontal axis at the base of the structure (33, 110, 210, 310, 410) and injecting the oxidant thereby to contact the combustible material (3, 220, 320, 420) at multiple points in a sequence; (e) controlling the product gas flow rate and composition by varying a flow rate of oxidant and a rate of movement of the oxidant injection point such that the combustible material (3, 220, 320, 420) is partly converted, thereby leaving behind an unconverted, thermally affected layer of combustible material (3, 220, 320, 420) in the sealed containment structure (33, 110, 210, 310, 410) after the injection point has passed through the material, wherein the conversion of the combustible material (3, 220, 320, 420) to a gas at one point in the sequence is initiated prior to complete conversion of the combustible material (3, 220, 320, 420) at a previous point in the sequence; f) cooling and purging the sealed containment structure (33, 110, 210, 310, 410) and loading fresh combustible material (3, 220, 320, 420) on top of the thermally affected layer left behind after completing steps c) to e); and g) repeating Steps b) to f).