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Christopher Dennis -- a street performer known for portraying Superman on Hollywood Blvd. for more than 25 years -- died from a lack of oxygen after crawling into a clothing donation bin while high on drugs.

The Man of Steel impersonator suffocated to death as a result of blunt force trauma to his head and neck ... this according to the L.A. County Coroner's Office. The Coroner's report also cites methamphetamine toxicity as a significant factor in his death.

Christopher suffered the fatal injuries after being partially entrapped within a metal box used for donating clothes, according to the Coroner's Office, which ruled his death accidental.

As we reported ... Christopher -- who was homeless -- died back in November when his lifeless body was discovered lying headfirst in the donation bin. Investigators believed he was attempting to gather clothes for himself.

Christopher entertained folks in Hollywood for over two decades by dressing up as Superman ... he also starred in the documentary "Confessions of a Superhero," and made multiple appearances on "Jimmy Kimmel Live!"