Now I upgraded the program.. thanks to all your help!! but i have a last question
how can I set the range or the stopper to T2 to 1000K because the table of the ideal
gas is for only 300K to 1000K so beyond that range in not applicable..

Code:
//This program generates a table for the specific internal energy
//and the specific enthalpy of Methane from 300K to 1000K using the
//specific heat variations listed on Table A.21

#include <stdio.h>
#include <conio.h>
#include <math.h>
#include <stdlib.h>
#include <string.h>

int main(){
  int QA;
  float T1=300,T2,end1,inc;
  double a=3.826,b=-3.979,c=24.558,d=-22.733,e=6.963;
  double A,B,C,D,E,H,U,R=8.3143,M=16.04,i=10;
  char error[128];
  
  do{
  do{
    system("cls");
    printf("Enter the Increment in Temperature: ");
    scanf("%s",&error);
    inc=atof(error);
    if(inc<=0){
      system("cls");
      printf("Invalid Input. Please try again!!!\n");
      printf("Please press the spacebar to continue!!");
      getche();
      }
    }while(inc<=0);
  T2=T1+inc;
  end1=1000-inc;
  printf("The increment is %.3f\n\n",inc);
  printf("\nInitial Temperature\tFinal Temperature\tEnthalpy\tInternal Energy\n");
  while(T1<=end1){
    T1+=inc;
    T2+=inc;
    A=a*(T2-T1);
    B=(1/pow(i,3))*(b/2)*(pow(T2,2)-pow(T1,2));
    C=(1/pow(i,6))*(c/3)*(pow(T2,3)-pow(T1,3));
    D=(1/pow(i,9))*(d/4)*(pow(T2,4)-pow(T1,4));
    E=(1/pow(i,12))*(e/5)*(pow(T2,5)-pow(T1,5));
    H=(R/M)*(A+B+C+D+E);
    U=(R/M)*((A+B+C+D+E)-(T2-T1));
    printf("%10.3f K\t\t%10.3f K\t\t%.3f kJ/kg\t%.3f kJ/kg\n",T1,T2,H,U);
    }
  do{//This used to ask the user if s/he wants to repeat the evaluation
    printf("\n\nDo you want to repeat the computation or Quit?\n");
    printf("[1] Yes\n");
    printf("[2] Quit\n");
    scanf("%s",&error);
    QA=atoi(error);
    switch(QA){
      case 1:
        break;
      case 2://Return to the selection screen
        exit(1);
        break;
      default://The program redirects invalid inputs to this part
        system("cls");
        printf("Invalid input. Please try again!!!\n");
        printf("Please press the spacebar to continue!!");
        getche();
        break;   
        }
      }while(QA>2||QA<1);
    }while(QA==1);         
  getche();
}