I used Navigator.push up to 6 screens to get to the payment page. After Payment, I want to push to the "Payment Successful" page then remove all the previous screens i.e using the back button will return to the very first screen.
NOTE: I have tried pushReplacementNamed and  it doesn't work.
Navigation in Flutter is done by Navigator class. Because the history is arranged in a stack structure, so moving to another screen means to launch a new instance of that full-screen widget. We will use push() for that.
I figured it out. It was the Navigator.pushAndRemoveUntil function. Where i had to pass the PaymentSuccessful widget as the newRoute, and the "/Home" route as the predicate
  _navPaymentSuccessful(){
    Navigator.pushAndRemoveUntil(
      context, 
      MaterialPageRoute(
        builder: (context) => PaymentSuccessful()
      ), 
     ModalRoute.withName("/Home")
    );
  }
Accepted Answer is correct. But you can try this too.
Navigator.pushAndRemoveUntil<dynamic>(
        context,
        MaterialPageRoute<dynamic>(
          builder: (BuildContext context) => YourPageNameGoesHere(),
        ),
        (route) => false,//if you want to disable back feature set to false
);
even simpler and I think a better way would be to do it this way,
this Schedules a callback for the end of the current persistent frame,to push to route /loginPage and removes all the previous routes,this way you can make sure that all the frames are rendered and then you navigate to next page. 
 SchedulerBinding.instance.addPostFrameCallback((_) {
                Navigator.of(context).pushNamedAndRemoveUntil(
                    '/loginPage', (Route<dynamic> route) => false);
              });
I would Suggest use WillPopScope in your Payment successful page and onWillPop method write following snippet of code:
 return WillPopScope(
      onWillPop: (){
        Navigator.of(context)
            .pushNamedAndRemoveUntil('/Home', (Route<dynamic> route) => false);
      },
      child: Scaffold()
};
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