How long is the shuttle bus ride from Beijing capital airport to south railway station

It takes about an hour.

Hope you both don't mind I jump in. If I want to go to Tianjin International Cruise (Xingang) port, what do I have to take? Do I take the shuttle bus ride to BJ South railway station then take bullet train to Tianjin station or Tanggu station? Or is there a better way? Thanks

Yes, you can take shuttle bus line 10 to BJ South Railway Station, where you can take bullet intercity to Tanggu. Upon your arrival at tanggu train station, you can take bus no. 102 to the port. It should be the best way to go.