NOVELTY - An off-gas stream (60) comprising hydrogen cyanide, potassium trihydrogen, hydrogen sulfide, carbon dioxide and optionally carbonyl sulfide and/or carbon disulfide is provided in sour slurry stripper, to obtain off-gas stream (60) and stripped slurry stream (70). Then, off-gas stream (60) is passed to hydrolysis zone to hydrolyze hydrogen cyanide and carbonyl sulfide or carbon disulfide and to obtain off-gas stream (110) which is scrubbed with aqueous acidic stream (180), to obtain off-gas stream (160). Thus, treatment of primary off-gas stream in a gasification process is performed.    USE - Treatment of off-gas stream in gasification process used for producing ammonium for fertilizer product.    DETAILED DESCRIPTION - An off-gas stream (60) comprising hydrogen cyanide, potassium trihydrogen, hydrogen sulfide, carbon dioxide and optionally carbonyl sulfide and/or carbon disulfide by separating a slurry bleed stream comprising particulate solids, hydrogen cyanide, ammonia, hydrogen sulfide, carbon monoxide and optionally carbonyl sulfide and carbon disulfide is provided in a sour slurry stripper, to obtain off-gas stream (60) and a stripped slurry stream comprising particulate solids. Then, the off-gas stream (60) is passed to a hydrolysis zone operating at pressure of 1.5-10 bar, to hydrolyze hydrogen cyanide and carbonyl sulfide or carbon disulfide, to obtain off-gas stream (110) comprising ammonia, hydrogen sulfide and carbon dioxide. The resultant off-gas stream (110) is scrubbed with an aqueous acidic stream in an ammonia scrubber (150), to obtain off-gas stream (160) comprising hydrogen sulfide and carbon dioxide and an ammonium-rich aqueous stream. Thus, treatment of primary off-gas stream in a gasification process is performed. An INDEPENDENT CLAIM is included for apparatus for treating off-gas stream (60) which comprises a sour slurry stripper to separate particulate material from slurry bleed stream, and has a primary inlet for the slurry bleed stream, a primary outlet for off-gas stream (60) and a secondary outlet for a stripped slurry stream comprising particulate solids, a hydrolysis zone to hydrolyze hydrogen cyanide and carbonyl sulfide and/or carbon disulfide in the off-gas stream (60), hydrolysis unit having a primary inlet for the off-gas stream (60) connected to the primary outlet of the sour slurry stripper and primary outlet for off-gas stream (110) and an ammonia scrubber to separate ammonia from the off-gas stream (110), where the ammonia scrubber has a primary inlet for the off-gas stream (110) connected to the primary outlet of the low pressure hydrolysis unit and a primary outlet for off-gas stream (160), secondary inlet for aqueous acidic stream and secondary outlet for ammonium-rich stream (170).    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view explaining treatment of off-gas stream in gasification process.    Off-gas stream (60,110,160,270)    Stripped slurry stream (70)    Off-gas stream (110)    Ammonia scrubber (150)    Ammonium-rich stream (170)    Aqueous acidic stream (180)    Hydrogen sulfide-comprising aqueous stream (260)    Flash vessel (350)    Bio-reactor (400)    Regenerated aqueous alkaline stream (420)    Solid/liquid separator (450)    Aqueous alkaline stream (460)    Sulfur slurry stream (470)