Rollercoaster time! The 737-800 is quite adept at climbing fast, even with a fuel passenger and fuel load!
Speaking of which, what are the usual climb rates for commercial airliners?
Usually under 3,000 fpm. Airliners don't takeoff or climb at wide open throttle, they used reduced power settings for fuel efficiency and reduce wear on the engines. Often times takeoff power is less than 95%,N1 where full throttle will get you 107% N1. That would explain your 6,000 fpm climb rates.
