504 error in Nginx+Passenger for long running Rails processes

We have a file import Rails process that runs for a long time. Since switching to Nginx from Apache, we began getting 504 timeout errors after exactly 10 minutes each time -- we don't have a 10 minute limit set anywhere in our nginx.conf. I've looked at several questions/answers on stackoverflow but haven't found a solution. I don't want to go back to Apache since we love Nginx's simplicity and speed but this is driving me crazy since I've tried every timeout config option in Nginx that I could find :-)We have a file import Rails process that runs f