Simplify the following text for a Grade 9 native English-speaking person.
Original: Midway Airport is the second largest passenger airport in the Chicago metropolitan area , as well as the state of Illinois , after Chicago O'Hare International Airport .
Simplified: Chicago Midway International Airport ( airport code KMDW ) is a secondary airport serving the city of Chicago , Illinois . It was the largest airport in Chicago before O'Hare International Airport was built .