Similes rather than metaphors predominate in the description of the gassed soldier; the central one, as the previous post identifies, is His hanging face, like a devil's sick of sin as he lies in the wagon containing the dead and dying.
It is crucial not just to spot similes but to give explanations of how they work. This contains no less than 3 other poetic techniques! Sick of sin is alliterative and there is assonance allowing one to almost spit out the words. Most importantly there is also contradiction, oxymoron: how can a devil be sick of sin? The answer is that it is only his face which is like a devil's, made hideous by the effects of the gas. He himself is a man, sick of the evil of War.
It is vitally important to remember that a simile is a figure of speech that compares one object or character with another object that we would not normally link it with by using the words "like" or "as." It is not to be confused with a metaphor, that does the same thing, but asserts a direct comparison without the bridging of the words "like" or "as." Thus when we consider the graphic and repulsive description of the soldier's face we can see that the simile that Owen uses is as follows:
And watch the white eyes writhing in his face,
His hanging face, like a devil's sick of sin...
Thus the face of the gassed soldier is compared to "a devil's sick of sin" which effectively describes the almost unnatural and strong effect of the gas on the dead soldier's body. Note how it is effective in the strong way it describes the face of the soldier. The reference to the devil only serves to reinforce Owen's central point about how wrong war is.