I can set the title of terminal using "set Title", but that title is not visible when the terminal is minimalized. the default title is being shown on the taskbar(where all open applications are visible). How to change that title?
In the profile's settings (Edit > Profile preferences), on tab Title and Command, set the default title and make sure to select Keep initial title in the combobox: .
For multiple titles, use Gnome Terminal's
If this won't work, I'll need more info on your configuration, especially what version of Ubuntu and what desktop environment/session (e.g. Gnome Classic) you use - screenshot showing the non-changed title in taskbar would be handy too.