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1. A process for producing a product gas stream rich in methane with a defined target pressure from a synthesis-gas fresh gas stream containing carbon oxides and hydrogen, comprising the following process steps:
(a) providing a synthesis-gas fresh stream at an inlet pressure, (b) combining the synthesis-gas fresh gas stream with a recirculation stream to obtain a synthesis-gas feed stream, (c) heating up the synthesis-gas feed stream and supplying the same to a main reaction zone, (d) converting the heated synthesis-gas feed stream to an intermediate-product gas stream enriched in methane in a main reaction zone under methanation conditions, wherein the main reaction zone includes at least one catalyst bed containing a methanation catalyst, (e) withdrawing a partial stream of the intermediate-product gas stream rich in methane after the main reaction zone as recirculation stream, wherein the recirculation stream is recirculated before the main reaction zone by means of a cycle compressor and is combined with the synthesis-gas fresh gas stream to obtain the synthesis-gas feed stream, (f) supplying the fraction of the intermediate-product gas stream rich in methane remaining after step (e) to a post-reaction zone, (g) converting the intermediate-product gas stream supplied to the post-reaction zone under methanation conditions to a product gas stream rich in methane, wherein the post-reaction zone includes at least one catalyst bed containing a methanation catalyst, (h) withdrawing the product gas stream rich in methane from the post-reaction zone and adjusting the target pressure by means of a product compressor, wherein an additional compressor is provided upstream of the main reaction zone and/or the post reaction zone.
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