Well, this is the first mention of "an arduino operated methane detector", so I guess that justifies the arduino.
You now need to look at two approaches
- Send data to phone and not the monitor
Disconnect USB cable, connect bluetooth to Pins 0,1, and install a bluetooth terminal on the phone. You might find Bluetooth Graphics Terminal a good choice. Since you have already got a result with serial monitor, no changes to your current programme are required.
- Send data to both phone and monitor.
This required additional software to enable you to use two serial ports, one for bluetooth and one for monitor. If you use a Uno, this means you need to use software serial.
I suggest you try option 1 first.
You might find the following background notes useful