A method of constructing an inner thermal lining or jacket of a jacketed gasifier having an outer shell with an opening allowing access to an interior of the gasifier, includes inserting jacket wall segments (40) into the gasifier interior through the opening, the jacket wall segments (40) each comprising an elongate jacket plate (42) with an annulus face (44) and a plurality of transversely extending longitudinally spaced stiffener formations (46) standing proud of the annulus face (44). The jacket wall segments (40) are arranged side by side leaving an aperture or space between adjacent jacket plates. The stiffener formations (46) of adjacent spaced jacket wall segments are welded together through the aperture or space between said adjacent spaced jacket wall segments, and the apertures or spaces are closed with window plates.