Treating a synthesis gas includes generating a plasma jet from a non-transferred arc torch having a main axis, the jet having a propagation axis substantially collinear with the torch main axis. The plasma torch is mounted on a feed enclosure. The syngas is received at an inlet port of the feed enclosure, downstream from the plasma torch and feeding the syngas so the flow encounters the plasma jet to mix the syngas and plasma jet in a distribution chamber. The mixture is propagated in a reactor downstream from the feed enclosure to convert the syngas into an outlet gas. The reactor is in communication in its upstream portion with the feed enclosure through a flared segment, and has a longitudinal axis that is substantially collinear with the propagation axis of the plasma jet. The outlet gas is extracted via an outlet port and particles are captured by a submerged conveyor.