It happened around 11 a.m. at an apartment complex in the 9900 block of Richmond Ave.
Fire officials said they are looking at a boiler as a cause of the carbon monoxide, but haven't yet confirmed that was the cause of the death.
People inside the complex called about a gas leak Saturday morning and during the evacuation, fire officials found the man dead.
Fire officials said some people have complained of the fumes for days.
Gas service is shut down to the entire complex and officials cannot turn it back on until they are sure the leak is sealed, officials said. That, as temperatures are expected to dip to near freezing Saturday night.