Q: How can I use my smart tv as a monitor for my Mac tower?

If you do not want do directly connect the iMac to the TV then yo can use an Apple TV and use AirPlay.

Use AirPlay to display video from your Mac on an HDTV - Apple Support

• Apple TV (2nd generation or later) with the latest Apple TV software
• For mirroring your display or extending your desktop, any of the following Mac computers with OS X Mountain Lion v10.8 or later:
  • iMac (Mid 2011 or later)
  • Mac mini (Mid 2011 or later)
  • MacBook (Early 2015 or later)
  • MacBook Air (Mid 2011 or later)
  • MacBook Pro (Early 2011 or later)
  • Mac Pro (Late 2013 or later)

If y have an older Mac:

http://www.macupdate.com/app/mac/36263/airplayer

There are lots of ways to do what you want. Some are more complex than others.

Have you tried connecting a cable directly from the Mac to one of the inputs on the TV? What inputs does it have?

What graphics output ports do you have on your iMac?

an iMac is not considered a Mac tower. The Mac Pro silver tower computers have no default display, and weigh 42lbs by themselves.