A process for producing a purified synthesis gas stream from a feed synthesis gas stream comprising besides the main constituents carbon monoxide and hydrogen also hydrogen sulphide, HCN and/or COS, the process comprising the steps of: (a) contacting one part of the feed synthesis gas stream with a water gas shift catalyst in a shift reactor in the presence of water and/or steam to react at least part of the carbon monoxide to hydrogen and carbon dioxide, to obtain a shifted synthesis gas stream enriched in hydrogen and in carbon dioxide; (b) contacting another part of the feed gas stream with a hydrolysis catalyst in the presence of water to remove HCN and/or COS to obtain a hydrolysed synthesis gas stream depleted in HCN and/or COS; (c) removing H2S from the shifted synthesis gas stream and from the hydrolysed synthesis gas stream by contacting these gas streams in a H2S-removal zone with an aqueous alkaline washing liquid to obtain a H2S-depleted synthesis gas stream and a sulphide-comprising aqueous stream; (d) contacting the sulphide-comprising aqueous stream with sulphide-oxidizing bacteria in the presence of oxygen in a bioreactor to obtain a sulphur slurry and aregenerated aqueous alkaline washing liquid; (e) removing carbon dioxide from the H2S-depleted synthesis gas stream, to obtain the purified synthesis gas stream.