I looked at tonight's flights on Flightaware,com.

The inbound from MSN arrives at Gate B15. Once exiting the plane, you've got about a 5 minute walk to the escalators that take you down to the underground transportation corridor which features moving walkways and a psychedelic light show. That will take you 5-7 minutes. The connection to LGA leaves from Gate A72. You've got two choices: walk and ride on the moving walkways, which for me would be 8-10 minutes, or go up an escalator to the red tram station (this is opposite gate A40), which has a maximum 4 minute wait and about a 2 minute ride. You would be about 2-3 minutes from your gate when you reach the north end of the tram line.

Bottom line is that you should make the connection in 15-20 minutes time at the outside. You should be OK.