Leroy Charles, 45, died Saturday night in his home in the South Chicago neighborhood. His wife and daughter found him after going to check on him.
An autopsy determined Charles died after breathing exhaust fumes from his furnace. The carbon monoxide level inside the house was 60 times the normal level.
Representatives at People's Gas say the company had turned off gas to the home and that someone else turned it back on incorrectly.
The victim's daughter is urging to people to be careful.
"Just check your carbon monoxide detectors and make sure they work," she said tearfully.
It was not clear Sunday night why the gas had been turned off.