PLENTY of time. You will not have to go through customs. Once you get off the JAL flight just follow the signs to connections and then to the NW transfer counter to get your boarding pass for your flight to dtw. You won't be able to get your boarding pass in ICN.

If you have any baggage you should be able to get it tagged at ICN all the way through to your final destination. Just give JAL your confirmation for the NW flight. Make sure you mention your bags when you get your boarding pass for NW at NRT as well.