Evacuations slides were used to remove passengers and crew members from the plane, the airline said.
The plane had 139 passengers and five crew aboard.
Southwest Airlines canceled all of its 220 flights from Chicago's Midway International Airport on Sunday due to a shortage of de-icing fluid.
At Midway Airport, average flight delays of more than 15 minutes were reported on Sunday afternoon and a total of 264 flights on all airlines were canceled. The airline said flight 2123 was pushing back from the gate when the ground crew observed a possible fire near the rear exterior of the aircraft. While a supply of the de-icing liquid arrived early Monday, a "handful of flights" may still be canceled, she said.
As of Monday morning, "Southwest has resumed operations at Chicago (Midway) and expects to operate a close to normal schedule", a spokesperson for the airline confirmed to Travel + Leisure.
According to the airport's Twitter account, the fire, which was apparently in the auxiliary power unit, has already been extinguished, while operations are "back to normal".
The US's third-largest city has recorded measurable snow - when at least 0.1 of an inch occurs - for nine consecutive days between 3 and 11 February, matching a record last set in January 2009.