This thread was unnecessary.
You answered and fixed your own problem, and knew what was causing your problem, yet you want someone to confirm what you already know or could have googled?
Sorry I'm a little rude? Maybe, sorry.
Just these are the threads that make smart helpful people, not even want to help people anymore, because they can't even help themselves.
Which in time, leads to no one wanting to help anyone because everyone expects someone to do research for them. This hurts the people that actually need help later down the road, because you cried wolf (In a sense) so much that they won't even think about helping.
Just my two cents =D
Now to the topic.
Did you really think a 600W in a closet that isn't open and no passive intake/outake would be okay for more than an hour?
That 600W right now is acting as a heater for the closet, no hot air out, no cold air in, no real air circulating, its just recycled air that gets hotter and hotter.
So to answer your question, yes.
Either crack the door open half a inch (Makes a huge difference trust me)
or cut open a 6.5 Inch hole at the top of the closet door, and insert some ducting, and exhaust using the inline.
You want to SUCK out the air from the closet, not blow air out. (Hope that made sense)
To give you an idea of how I overcame my heat problem, it was extremely simple.
My tent temps were hitting 100F+, this was because I was pushing in more air than I was pushing out, causing the air to just heat up instantly as it meshed with the hotter air still inside.
Instead of opening the tent door to let air in or only use passive intake/outake I hooked up ducting to my air cooled hood, and put the Inline on the exit end of the ducting and sucked air from out the hood to outside.
Now the other end of the air cooled hood was open.
Two birds, one stone.
Sucking hot air out of the hood, and sucking hot air out the tent as well. Causing a negative pressure that sucked in fresh air from the passive flaps.
Now I'm cruising on 80F with 2 400Ws and no A/c yet.